This invention relates to the formation of masks useful in photolithography for the fabrication of semiconductor devices, and more particularly, to a construction of masks having regions of material of different optical opacity, the regions being arranged typically as a succession of layers.
In the use of photolithography for the construction of semiconductor devices as well as other devices such as masks and reticles, the construction process involves numerous steps which include the exposure of photoresist through a mask to delineate specifically shaped areas which are to be etched. There are situations in which a plurality of exposure steps are to be performed sequentially with a plurality of masks to accomplish differing amounts of exposure of the various areas to be etched.
The manufacturing process can be simplified by reduction of the number of exposure steps by use of a gray level mask. A gray level mask allows a defining of two or more conventional mask levels in a single exposure step. This technique lends itself to process clustering with its potential for low defect density, and also facilitates manufacture by reducing the number of masking levels required. The technique also reduces tolerances between gray level patterns. By way of example, an important use of gray level mask technology is for recessed multilevel wiring applications where via and wiring patterns may be produced with one exposure.
A problem arises with presently available gray level masks in that the masks are difficult to fabricate and, furthermore, produce inadequate image quality in many applications. By way of example in the fabrication of a gray level mask, it has been the practice to form the mask by an array of spaced-apart chromium islands in the nature of a half-tone screen wherein each chromium island is opaque to ultraviolet radiation while spaces between the islands allow passage of the radiation. The half-tone screen is constructed by use of electron-bream lithography so as to produce spaces between the islands wherein the spaces have dimensions smaller than a wavelength of the optical radiation. The islands may also have dimensions smaller than the wavelength of the optical radiation. As a result, there is a significant attenuation of the optical radiation transmitted through the mask. The resulting transmissivity of the mask is significantly more than that of a totally opaque mask region and significantly less than that of a totally transparent mask region. Thus, the resulting mask is a gray level mask, but a mask which produces a lower quality image than is desired. The amount of transmissivity is defined by the dimensions of the chromium islands and the spaces.
A further disadvantage of the foregoing gray level mask is the fact that images formed in the gray areas have sloped sidewalls which are unacceptable for use in producing semiconductor products requiring the higher resolution for condensed packaging of circuit elements as are being contemplated for the near future. With respect to other techniques which have been employed in the fabrication of gray level masks, there has been the disadvantage that the other fabrication methods require precise electron-beam dose control to make optical masks or electron-beam proximity correction, and produce gray layers of specific opacity.